A power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) includes a device region for passing current, and a termination region formed around the device region in the outer peripheral portion of the chip. In order to prevent destruction of the power MOSFET when avalanche breakdown occurs in the drift layer of the power MOSFET, carriers generated by avalanche breakdown need to be ejected from the device region side to a source electrode. The device region has a larger cross-sectional area for ejecting carriers to the source electrode than the termination region. Hence, the device region has lower ejection resistance, and can prevent device destruction due to current concentration. Thus, it is desirable that the breakdown voltage of the device region be set lower than that of the termination region.
Furthermore, to increase the breakdown voltage of the power MOSFET, the drift layer needs to be a high resistance layer with low impurity concentration. However, because the on-resistance is desirably reduced in the device region, the drift layer is desired to be a low resistance layer with high impurity concentration. Thus, there is a tradeoff between the breakdown voltage and the on-resistance of the power MOSFET. To improve this tradeoff, a super junction structure is used in the drift layer of the power MOSFET. By using a super junction structure in the drift layer, the breakdown voltage of the power MOSFET can be increased while increasing the impurity concentration of the current path.
The drift layer of the power MOSFET is designed so as to include a super junction structure in the device region and a high resistance layer in the termination region. The super junction structure can reduce the on-resistance while maintaining high breakdown voltage. The high resistance layer has high resistance, but its breakdown voltage is even higher. In the power MOSFET with such structure, avalanche breakdown occurs in the device region rather than in the termination region. Hence, this power MOSFET has the characteristics of high avalanche withstand capability, low on-resistance, and high breakdown voltage.